There are approximately 200,000 hip replacement surgeries each year in the U.S. alone and about 800,000 worldwide. The number of knee replacement surgeries is approximately double this amount. In the future, the number of hip, shoulder, disk and knee replacement surgeries will continue to grow exponentially as the population ages.
Hip replacement surgery entails surgical intervention of two parts of the hip joint, the acetabulum (a cup-shaped bone in the pelvis) and the femoral head (the ball-shaped end of the thigh bone), then replacing each with smooth artificial surfaces. Shoulder replacements are similar, with knee and disk replacements being somewhat more complex. Exemplary artificial surfaces are high-density plastic, metal, and ceramic materials.
Hip replacements are used illustratively herein because they are most frequently carried out. Total hip replacements are most often performed for severe painful arthritic conditions but sometimes are performed for other problems such as deformities, fractures, tumors, or aseptic necrosis of the bone. In the majority of cases, hip replacement surgery is very successful in relieving pain, restoring function, and markedly improving the quality of life for patients with hip disease. The caveat is that the average life of a prosthetic implant is only about ten years. This relatively short functional existence is due to problems with decomposition (wear) of the prosthetic's fabrication materials and loosening of the joint due to osteolysis (bone loss) and other complications such as fatique and degeneration of the artificial joint.
Wear debris and the resulting osteolysis from inflammatory cellular responses to wear debris are the most significant factors contributing to failure of hip replacements. Osteolysis, when undetected and untreated, can result in massive bone loss and implant failure.
Joint revision (replacement) procedures can be more difficult than the original surgery due to diseased, damaged, and decomposed bone in the area of the former prosthesis. Often there is little original natural bone remaining to attach a new prosthesis after the damaged prosthesis is removed. The magnitude of this problem is significant. For example, in 1996, total hip revision rates were about one third of the primary hip replacements. The function and long term survival of revision total hip arthroplasty is generally inferior to primary hip arthroplasty and leads to a worsened quality of life for many patients.
The amount of wear in a hip prosthesis is dependent upon many factors, including the materials used in fabrication, the weight of the patient, the age of the patient, and the patient's activity level, to name just a few. As mentioned above, materials commonly utilized in fabrication of hip prosthetics include plastic (ultra high molecular weight polyethylene, UHMWPE), metal (titanium or cobalt-chromium alloy), or alumina and zirconia ceramics.
The amount of wear for polymeric materials is in the region of 50-100 mm3 per year for UHMWPE and 10 mm3 (or smaller) per year for the newest cross-linked UHMWPE. The wear debris comes from the grinding of the metal against the polymer as the patient moves the prosthetic hip. Metal on metal and ceramic on ceramic implants sometimes have less wear debris than plastic implants; however they come with other risks, including suffering adverse biological effects to increased metal ions in the body, experiencing chipping of ceramic components, and risking fracture of the implant. Also, with metal and ceramic implants there are additional hurdles to overcome in fabrication such as the difficulty in obtaining conforming surfaces and consistently correct clearances. Lastly, wear debris from a prosthesis can also lead to bacterial infection in the bone causing failure of the prosthesis.
Current in vivo methods for measuring wear of a prosthetic implant are limited to radiological surveying and physical examination of the patient. However, in practice, both of these methods are truly useful only for detecting painful, catastrophic prosthetic failure, such that which happens in implant fracture or severe loosening from massive bone loss. Lesser amounts of implant decomposition are very difficult to diagnose due to the modest degree of physical symptoms.
In addition to wear, artificial joints, particularly those made of UHMWPE, a preferred artificial joint material, also degrade and fatigue due to free radical reactions that can occur prior to implantation and also in vivo, during use. Free radicals are typically generated prior to implantation by gamma irradiation of the artificial joint during sterilization, even when the sterilization is carried out in the substantial absence of oxygen. Free radicals are also generated in vivo due to mechanical chain scission or chemically by the action of blood- or serum-borne oxidants.
As mentioned before, radiology often underestimates the amount of wear due to implant decomposition. In addition, the method is imprecise, and as such, is not reliable for diagnosing less than component failure and/or catastrophic loss of bone stock. The reason for this unreliability is the physical difficulty in measuring small incremental changes that the prosthetic components may have made by X-ray methods. It is also difficult to standardize radiographs. In addition, there are health risks such as cancer due to the patient repeatedly being X-rayed multiple times. Furthermore, in polyethylene prosthetics, fluid absorption and/or creep of the polyethylene samples can be in the same order of magnitude as the wear itself. For these reasons, evaluation of in vivo wear of prosthetic implants, especially knee replacements, has been most precisely diagnosed from surgically removed implants.
Thus, there is an urgent need for a process for early detection and quantification of prosthetic implant wear. It is desirable that such a process be accurate so as to address smaller changes in prosthetic wear reliably. It is also desirable that the process be safe for the patient. The invention described hereinafter not only provides such a process but a novel plastic prosthesis that contains a tracer to facilitate such a process.